Sarcomere alignment is regulated by myocyte shape

Cardiac organogenesis and pathogenesis are both characterized by changes in myocyte shape, cytoskeletal architecture, and the extracellular matrix (ECM). However, the mechanisms by which the ECM influences myocyte shape and myofibrillar patterning are unknown. We hypothesized that geometric cues in the ECM align sarcomeres by directing the actin network orientation. To test our hypothesis, we cultured neonatal rat ventricular myocytes on islands of micro-patterned ECM to measure how they remodeled their cytoskeleton in response to extracellular cues. Myocytes spread and assumed the shape of circular and rectangular islands and reorganized their cytoskeletons and myofibrillar arrays with respect to the ECM boundary conditions. Circular myocytes did not assemble predictable actin networks nor organized sarcomere arrays. In contrast, myocytes cultured on rectangular ECM patterns with aspect ratios ranging from 1:1 to 7:1 aligned their sarcomeres in predictable and repeatable patterns based on highly localized focal adhesion complexes. Examination of averaged alpha-actinin images revealed invariant sarcomeric registration irrespective of myocyte aspect ratio. Since the sarcomere sub-units possess a fixed length, this observation indicates that cytoskeleton configuration is length-limited by the extracellular boundary conditions. These results indicate that modification of the extracellular microenvironment induces dynamic reconfiguring of the myocyte shape and intracellular architecture. Furthermore, geometric boundaries such as corners induce localized myofibrillar anisotropy that becomes global as the myocyte aspect ratio increases.     

Self-organization of muscle cell structure and function

The organization of muscle is the product of functional adaptation over several length scales spanning from the sarcomere to the muscle bundle. One possible strategy for solving this multiscale coupling problem is to physically constrain the muscle cells in microenvironments that potentiate the organization of their intracellular space. We hypothesized that boundary conditions in the extracellular space potentiate the organization of cytoskeletal scaffolds for directed sarcomeregenesis. We developed a quantitative model of how the cytoskeleton of neonatal rat ventricular myocytes organizes with respect to geometric cues in the extracellular matrix. Numerical results and in vitro assays to control myocyte shape indicated that distinct cytoskeletal architectures arise from two temporally-ordered, organizational processes: the interaction between actin fibers, premyofibrils and focal adhesions, as well as cooperative alignment and parallel bundling of nascent myofibrils. Our results suggest that a hierarchy of mechanisms regulate the self-organization of the contractile cytoskeleton and that a positive feedback loop is responsible for initiating the break in symmetry, potentiated by extracellular boundary conditions, is required to polarize the contractile cytoskeleton.     

Cooperative control of Akt phosphorylation, bcl-2 expression, and apoptosis by cytoskeletal microfilaments and microtubules in capillary endothelial cells

Capillary endothelial cells can be switched between growth and apoptosis by modulating their shape with the use of micropatterned adhesive islands. The present study was carried out to examine whether cytoskeletal filaments contribute to this response. Disruption of microfilaments or microtubules with the use of cytochalasin D or nocodazole, respectively, led to levels of apoptosis in capillary cells equivalent to that previously demonstrated by inducing cell rounding with the use of micropatterned culture surfaces containing small (<20 microm in diameter) circular adhesive islands coated with fibronectin. Simultaneous disruption of microfilaments and microtubules led to more pronounced cell rounding and to enhanced levels of apoptosis approaching that observed during anoikis in fully detached (suspended) cells, indicating that these two cytoskeletal filament systems can cooperate to promote cell survival. Western blot analysis revealed that the protein kinase Akt, which is known to be critical for control of cell survival became dephosphorylated during cell rounding induced by disruption of the cytoskeleton, and that this was accompanied by a decrease in bcl-2 expression as well as a subsequent increase in caspase activation. This ability of the cytoskeleton to control capillary endothelial cell survival may be important for understanding the relationship among extracellular matrix turnover, cell shape changes, and apoptosis during angiogenesis inhibition.     

Effect of cytoskeleton disruptors on L-type Ca channel distribution in rat ventricular myocytes

The cytoskeleton plays an important role in many aspects of cardiac cell function, including protein trafficking. However, the role of the cytoskeleton in determining Ca channel location in cardiac myocytes is unknown. In the present study we therefore investigated the effect of the cytoskeletal disruptors cytochalasin D, latrunculin, nocadazole and colchicine on the distribution of Ca channels in rat ventricular myocytes during culture for up to 96 h. During culture in the absence of these agents, cell edges became rounded, t-tubule density decreased, and the normal transverse distribution of the alpha1 (pore-forming) subunit of the L-type Ca channel became more punctate and peri-nuclear; these changes were associated with loss of synchronous Ca release in response to electrical stimulation. Disruption of tubulin using nocadazole or colchicine or sequestration of monomeric actin by latrunculin had no effect on these changes. In contrast, cytochalasin D inhibited these changes: cell shape, t-tubule density, transverse Ca channel staining and synchronous Ca release were maintained during culture. The protein synthesis inhibitor cycloheximide had similar effects to cytochalasin. These data suggest that cytochalasin stabilizes actin in adult ventricular myocytes in culture, thus stabilizing cell structure and function, and that actin is important in trafficking L-type Ca channels from the peri-nuclear region to the t-tubules, where they are normally located and provide the trigger for Ca release.     

Extracellular matrix controls myosin light chain phosphorylation and cell contractility through modulation of cell shape and cytoskeletal prestress

The mechanism by which vascular smooth muscle (VSM) cells modulate their contractility in response to structural cues from extracellular matrix remains poorly understood. When pulmonary VSM cells were cultured on increasing densities of immobilized fibronectin (FN), cell spreading, myosin light chain (MLC) phosphorylation, cytoskeletal prestress (isometric tension in the cell before vasoagonist stimulation), and the active contractile response to the vasoconstrictor endothelin-1 all increased in parallel. In contrast, MLC phosphorylation did not increase when suspended cells were allowed to bind FN-coated microbeads (4.5-microm diameter) or cultured on micrometer-sized (30 x 30 microm) FN islands surrounded by nonadhesive regions that support integrin binding but prevent cell spreading. Cell spreading and MLC phosphorylation also both decreased in parallel when the mechanical compliance of flexible FN substrates was raised. MLC phosphorylation was inhibited independently of cell shape when cytoskeletal prestress was dissipated using a myosin ATPase inhibitor in fully spread cells, whereas it increased to maximal levels when microtubules were disrupted using nocodazole in cells adherent to FN but not in suspended cells. These data demonstrate that changes in cell-extracellular matrix (ECM) interactions modulate smooth muscle cell contractility at the level of biochemical signal transduction and suggest that the mechanism underlying this regulation may involve physical interplay between ECM and the cytoskeleton, such that cell spreading and generation of cytoskeletal tension feed back to promote MLC phosphorylation and further increase tension generation.     

Subcellular curvature at the perimeter of micropatterned cells influences lamellipodial distribution and cell polarity

This paper employs substrates that are patterned with shapes having well-defined geometric cues to characterize the influence of curvature on the polarization of highly metastatic B16F10 rat melanoma cells. Substrates were patterned using microcontact printing to define adhesive islands of defined shape and size on a background that otherwise prevents cell adhesion. Cells adherent to these surfaces responded to local curvature at the perimeter of the adhesive islands; convex features promoted the assembly of lamellipodia and concave features promoted the assembly of stress filaments. Cells adherent to rectangular shapes displayed a polarized cytoskeleton that increased with the aspect ratio of the shapes. Shapes that combined local geometric cues, by way of concave or convex edges, with aspect ratio were used to understand the additive effects of shape on polarization. The dependence of cell polarity on shape was determined in the presence of small molecules that alter actomyosin contractility and revealed a stronger dependence on contractility for shapes having straight edges, in contrast to those having curved edges. This study demonstrates that the cytoskeleton modulates cell polarity in response to multiple geometric cues in the extracellular environment.     

Viscoelastic retraction of single living stress fibers and its impact on cell shape, cytoskeletal organization, and extracellular matrix mechanics

Cells change their form and function by assembling actin stress fibers at their base and exerting traction forces on their extracellular matrix (ECM) adhesions. Individual stress fibers are thought to be actively tensed by the action of actomyosin motors and to function as elastic cables that structurally reinforce the basal portion of the cytoskeleton; however, these principles have not been directly tested in living cells, and their significance for overall cell shape control is poorly understood. Here we combine a laser nanoscissor, traction force microscopy, and fluorescence photobleaching methods to confirm that stress fibers in living cells behave as viscoelastic cables that are tensed through the action of actomyosin motors, to quantify their retraction kinetics in situ, and to explore their contribution to overall mechanical stability of the cell and interconnected ECM. These studies reveal that viscoelastic recoil of individual stress fibers after laser severing is partially slowed by inhibition of Rho-associated kinase and virtually abolished by direct inhibition of myosin light chain kinase. Importantly, cells cultured on stiff ECM substrates can tolerate disruption of multiple stress fibers with negligible overall change in cell shape, whereas disruption of a single stress fiber in cells anchored to compliant ECM substrates compromises the entire cellular force balance, induces cytoskeletal rearrangements, and produces ECM retraction many microns away from the site of incision; this results in large-scale changes of cell shape (> 5% elongation). In addition to revealing fundamental insight into the mechanical properties and cell shape contributions of individual stress fibers and confirming that the ECM is effectively a physical extension of the cell and cytoskeleton, the technologies described here offer a novel approach to spatially map the cytoskeletal mechanics of living cells on the nanoscale.     

Signal transduction by integrin receptors for extracellular matrix: cooperative processing of extracellular information.

Adhesion receptors allow cells to interact with a dynamic and information-rich environment of extracellular matrix molecules. The integrin family of adhesion receptors transduces signals from the extracellular matrix that regulate growth, gene expression and differentiation, as well as cell shape, motility and cytoskeletal architecture. Recent data support the hypothesis that integrins transduce signals cooperatively with other classes of adhesion receptors or with growth factor receptors. Furthermore, the ability of integrins to interact with the cytoskeleton appears to be fundamental to their mechanism for signal transduction.     

Cell shape regulates collagen type I expression in human tendon fibroblasts

Understanding the relationship between cell shape and cellular function is important for study of cell biology in general and for regulation of cell phenotype in tissue engineering in particular. In this study, microcontact printing technique was used to create cell-adhesive rectangular and circular islands. The rectangular islands had three aspect ratios: 19.6, 4.9, and 2.2, respectively, whereas circular islands had a diameter of 50 microm. Both rectangular and circular islands had the same area of 1960 microm(2). In culture, we found that human tendon fibroblasts (HTFs) assumed the shapes of these islands. Quantitative immunofluorescence measurement showed that more elongated cells expressed higher collagen type I than did less stretched cells even though cell spreading area was the same. This suggests that HTFs, which assume an elongated shape in vivo, have optimal morphology in terms of expression of collagen type I, which is a major component of normal tendons. Using immunohistochemistry along with cell traction force microscopy (CTFM), we further found that these HTFs with different shapes exhibited variations in actin cytoskeletal structure, spatial arrangement of focal adhesions, and spatial distribution and magnitude of cell traction forces. The changes in the actin cytoskeletal structure, focal adhesion distributions, and traction forces in cells with different shapes may be responsible for altered collagen expression, as they are known to be involved in cellular mechanotransduction.     

Mechanical control of tissue morphogenesis during embryological development

Twenty years ago, we proposed a model of developmental control based on tensegrity architecture, in which tissue pattern formation in the embryo is controlled through mechanical interactions between cells and extracellular matrix (ECM) which place the tissue in a state of isometric tension (prestress). The model proposed that local changes in the mechanical compliance of the ECM, for example, due to regional variations in basement membrane degradation beneath growing epithelium, may result in local stretching of the ECM and associated adherent cells, much like a "run-in-a-stocking". Cell growth and function would be controlled locally though physical distortion of the associated cells, or changes in cytoskeletal tension. Importantly, experimental studies have demonstrated that cultured cells can be switched between different fates, including growth, differentiation, apoptosis, directional motility and different stem cell lineages, by modulating cell shape. Experiments in whole embryonic organ rudiments also have confirmed the tight correlation between basement membrane thinning, cell tension generation and new bud and branch formation during tissue morphogenesis and that this process can be inhibited or accelerated by dissipating or enhancing cytoskeletal tension, respectively. Taken together, this work confirms that mechanical forces generated in the cytoskeleton of individual cells and exerted on ECM scaffolds, play a critical role in the sculpting of the embryo.     

Patterning of the membrane cytoskeleton by the extracellular matrix

The extracellular matrices of different tissues contain components which affect the migration, morphology and differentiation of many types of cells. These forms of cell behavior often involve dramatic changes in cytoskeletal organization. Extracellular matrix components are recognized by specific cell surface receptors which span the membrane and interact with the actin cytoskeleton. In cultured cells, the matrix receptors are concentrated in sites of cell attachment called focal adhesions. Information that is conveyed from the extracellular matrix to the cytoskeleton may involve matrix components, cell surface receptors, as well as the proteins at the cytoplasmic face of the focal adhesion which link the receptors to the actin cytoskeleton.     

Activation of ROCK by RhoA is regulated by cell adhesion, shape, and cytoskeletal tension

Adhesion to the extracellular matrix regulates numerous changes in the actin cytoskeleton by regulating the activity of the Rho family of small GTPases. Here, we report that adhesion and the associated changes in cell shape and cytoskeletal tension are all required for GTP-bound RhoA to activate its downstream effector, ROCK. Using an in vitro kinase assay for endogenous ROCK, we found that cells in suspension, attached on substrates coated with low density fibronectin, or on spreading-restrictive micropatterned islands all exhibited low ROCK activity and correspondingly low myosin light chain phosphorylation, in the face of high levels of GTP-bound RhoA. In contrast, allowing cells to spread against substrates rescued ROCK and myosin activity. Interestingly, inhibition of tension with cytochalasin D or blebbistatin also inhibited ROCK activity within 20 min. The abrogation of ROCK activity by cell detachment or inhibition of tension could not be rescued by constitutively active RhoA-V14. These results suggest the existence of a feedback loop between cytoskeletal tension, adhesion maturation, and ROCK signaling that likely contributes to numerous mechanochemical processes.     

Bioassay development: The implications of cardiac myocyte motility in vitro

Summary Cardiac myocytes cultured over microfabricated extracellular recording devices can be used to assay bioactive compounds. However, electrophysiological signals recorded from these devices vary in amplitude with time. Theoretically, changes in signal amplitude arise from myocytes being moved over recording sites by cocultured fibroblasts. To test this, neonatal rat cardiac myocytes were cultured at high densities and low densities on fibronectin-coated glass. After 36.5 h, myocytes were identified by their rhythmic contractions and then time-lapse-recorded for 3.5 h. Length, width, and angle of orientation was then determined every 30 min for five cells in low density and five cells in high-density culture. Low-density cells had mean lengths of 65.3 μm and widths of 35.1 μm, whereas cells in high-density culture had greater mean lengths of 74.2 μm and lower mean widths of 24.3 μm. Length, width, and angle of orientation of cells in low- and high-density culture changed by 4.1%, 11.8%, and 2.7 degrees, and 6.4%, 10%, and 4.6 degrees, respectively, every half hour. We found no evidence of myocyte-fibroblast interactions influencing cell position or shape in low density, but in high density, we found evidence that fibroblast-myocyte interactions could transiently influence cell shape. We conclude that fibroblast-independent changes in cell shape are largely responsible for the changes in signal amplitude recorded from cardiac myocytes cultured on microfabricated extracellular recording devices. However, there is some evidence that myocyte-fibroblast interactions may augment this process in high-density culture. The implications of these findings for bioassay development are discussed.     

Increasing fibroblast response to materials using nanotopography: morphological and genetic measurements of cell response to 13-nm-high polymer demixed islands

It is becoming clear that cells can respond not only to micometric scale topography, but may also to nanometric scale topography. The production of reproducibly sized nanometric features has relied heavily on expensive and time-consuming methods of manufacture, such as electron beam lithography. Polymer demixing of polystyrene and polybromostyrene has been found to produce nanoscale islands of reproducible height, and the islands have been previously shown to effect cell spreading compared to planar surfaces. This study observes morphological, cytoskeletal, and molecular changes in fibroblast reaction to 13-nm-high islands. The methods employed include scanning electron microscopy, fluorescent microscopy, and 1718 gene microarray. The results show that the cells respond to the islands by broad gene up-regulation, notably in the areas of cell signaling, proliferation, cytoskeleton, and production of extracellular matrix proteins. Microscopical results provide confirmation of the microarray findings.     

Cardiac myocyte remodeling mediated by N-cadherin-dependent mechanosensing

Cell-to-cell adhesions are crucial in maintaining the structural and functional integrity of cardiac cells. Little is known about the mechanosensitivity and mechanotransduction of cell-to-cell interactions. Most studies of cardiac mechanotransduction and myofibrillogenesis have focused on cell-extracellular matrix (ECM)-specific interactions. This study assesses the direct role of intercellular adhesion, specifically that of N-cadherin-mediated mechanotransduction, on the morphology and internal organization of neonatal ventricular cardiac myocytes. The results show that cadherin-mediated cell attachments are capable of eliciting a cytoskeletal network response similar to that of integrin-mediated force response and transmission, affecting myofibrillar organization, myocyte shape, and cortical stiffness. Traction forces mediated by N-cadherin were shown to be comparable to those sustained by ECM. The directional changes in predicted traction forces as a function of imposed loads (gel stiffness) provide the added evidence that N-cadherin is a mechanoresponsive adhesion receptor. Strikingly, the mechanical sensitivity response (gain) in terms of the measured cell-spread area as a function of imposed load (adhesive substrate rigidity) was consistently higher for N-cadherin-coated surfaces compared with ECM protein-coated surfaces. In addition, the cytoskeletal architecture of myocytes on an N-cadherin adhesive microenvironment was characteristically different from that on an ECM environment, suggesting that the two mechanotransductive cell adhesion systems may play both independent and complementary roles in myocyte cytoskeletal spatial organization. These results indicate that cell-to-cell-mediated force perception and transmission are involved in the organization and development of cardiac structure and function.     

The cytoskeleton and epidermal morphogenesis in C. elegans

During Caenorhabditis elegans development, the process of epidermal elongation converts the bean-shaped embryo into the long thin shape of the larval worm. Epidermal elongation results from changes in the shape of epidermal cells, which in turn result from changes in the epidermal cytoskeleton, the extracellular matrix, and in cell-matrix adhesion junctions. Here, we review the roles of cytoskeletal filament systems in epidermal cell shape change during elongation. Genetic and cell biological analyses have established that all three major cytoskeletal filament systems (actin microfilaments, microtubules, and intermediate filaments (IFs)) play distinct and essential roles in epidermal cell shape change. Recent work has also highlighted the importance of communication between these systems for their integrated function in epidermal elongation. Epidermal cells undergo reciprocal interactions with underlying muscle cells, which regulate the position and function of IF-containing cell-matrix adhesion structures within the epidermis. Elongation thus exemplifies the reciprocal tissue interactions of organogenesis.     

Actin cytoskeleton dynamics and the cell division cycle

The network of actin filaments is one of the crucial cytoskeletal structures contributing to the morphological framework of a cell and which participates in the dynamic regulation of cellular functions. In adherent cell types, cells adhere to the substratum during interphase and spread to assume their characteristic shape supported by the actin cytoskeleton. This actin cytoskeleton is reorganized during mitosis to form rounded cells with increased cortical rigidity. The actin cytoskeleton is re-established after mitosis, allowing cells to regain their extended shape and attachment to the substratum. The modulation of such drastic changes in cell shape in coordination with cell cycle progression suggests a tight regulatory interaction between cytoskeleton signalling, cell–cell/cell–matrix adhesions and mitotic events. Here, we review the contribution of the actin cytoskeleton to cell cycle progression with an emphasis on the effectors responsible for the regulation of the actin cytoskeleton and integration of their activities with the cell cycle machinery.     

Control of Cyclin D1, p27Kip1, and Cell Cycle Progression in Human Capillary Endothelial Cells by Cell Shape and Cytoskeletal Tension

The extracellular matrix (ECM) plays an essential role in the regulation of cell proliferation during angiogenesis. Cell adhesion to ECM is mediated by binding of cell surface integrin receptors, which both activate intracellular signaling cascades and mediate tension-dependent changes in cell shape and cytoskeletal structure. Although the growth control field has focused on early integrin and growth factor signaling events, recent studies suggest that cell shape may play an equally critical role in control of cell cycle progression. Studies were carried out to determine when cell shape exerts its regulatory effects during the cell cycle and to analyze the molecular basis for shape-dependent growth control. The shape of human capillary endothelial cells was controlled by culturing cells on microfabricated substrates containing ECM-coated adhesive islands with defined shape and size on the micrometer scale or on plastic dishes coated with defined ECM molecular coating densities. Cells that were prevented from spreading in medium containing soluble growth factors exhibited normal activation of the mitogen-activated kinase (erk1/erk2) growth signaling pathway. However, in contrast to spread cells, these cells failed to progress through G1 and enter S phase. This shape-dependent block in cell cycle progression correlated with a failure to increase cyclin D1 protein levels, down-regulate the cell cycle inhibitor p27^Kip1, and phosphorylate the retinoblastoma protein in late G1. A similar block in cell cycle progression was induced before this same shape-sensitive restriction point by disrupting the actin network using cytochalasin or by inhibiting cytoskeletal tension generation using an inhibitor of actomyosin interactions. In contrast, neither modifications of cell shape, cytoskeletal structure, nor mechanical tension had any effect on S phase entry when added at later times. These findings demonstrate that although early growth factor and integrin signaling events are required for growth, they alone are not sufficient. Subsequent cell cycle progression and, hence, cell proliferation are controlled by tension-dependent changes in cell shape and cytoskeletal structure that act by subjugating the molecular machinery that regulates the G1/S transition.     

Myocyte cytoskeletal disorganization and right heart failure in hypoxia-induced neonatal pulmonary hypertension

Previous studies have demonstrated that environmentally or genetically induced changes in the intracellular proteins that compose the cytoskeleton can contribute to heart failure. Because neonatal right ventricular myocytes are immature and are in the process of significant cytoskeletal change, we hypothesized that they may be particularly susceptible to pressure stress. Newborn calves exposed to hypobaric hypoxia (barometric pressure = 430 mmHg) for 14 days developed severe pulmonary hypertension (pulmonary arterial pressure = 101 +/- 6 vs. 27 +/- 1 mmHg) and right heart failure compared with age-matched controls. Light microscopy showed partial loss of myocardial striations in the failing neonatal right but not left ventricles and in neither ventricle of adolescent cattle dying of altitude-induced right heart failure. In neonatal calves, immunohistochemical analysis of the cytoskeletal proteins (vinculin, metavinculin, desmin, vimentin, and cadherin) showed selectively, within the failing right ventricles, patchy areas characterized by loss and disorganization of costameres and intercalated discs. Within myocytes from the failing ventricles, vinculin and desmin were observed to redistribute diffusely within the cytosol, metavinculin appeared in disorganized clumps, and vimentin immunoreactivity was markedly decreased. Western blot analysis of the failing right ventricular myocardium showed, compared with control, vinculin and desmin to be little changed in total content but redistributed from insoluble (structural) to soluble (cytosolic) fractions; metavinculin total content was markedly decreased, tubulin content increased, particularly in the structural fraction, and cadherin total content and distribution were unchanged. We conclude that hypoxic pulmonary hypertensive-induced neonatal right ventricular failure is associated with disorganization of the cytoskeletal architecture.     

A bacterial linear motor: cellular and molecular organization of the contractile cytoskeleton of the helical bacterium Spiroplasma melliferum BC3

The Mollicutes (Mycoplasma, Acholeplasma, and Spiroplasma) are the smallest, simplest and most primitive free-living and self-replicating known cells. These bacteria have evolved from Clostridia by regressive evolution and genome reduction to the range of 5.8 x 10(5)-2.2 x 10(6) basepairs (bp). Structurally, the Mollicutes completely lack cell walls and are enveloped by only a cholesterol containing cell membrane. The Mollicutes contain what can be defined as a bacterial cytoskeleton. The Spiroplasmas are unique in having a well-defined, dynamic, helical cell geometry and a flat, monolayered, membrane-bound cytoskeleton, which follows, intracellularly, the shortest helical line on the cellular coil. By applying cryo-electron-microscopy to whole cells, isolated cytoskeletons and cytoskeletal fibrils and subunits, as well as by selective extraction of cellular components, we determined, at a resolution of approximately 25 A, the cellular and molecular organization of the cytoskeleton. The cytoskeleton is assembled from a 59 kDa protein. The 59 kDa protein, has an equivalent sphere diameter of approximately 50 A. Given the approximately 100 A axial and lateral spacings in the cytoskeletal ribbons and the near-circular shape of the subunit, we suggest that the subunit is a tetramer of 59 kDa monomers; the tetramers assemble further into flat fibrils, seven of which form a flat, monolayered, well-ordered ribbon. The cytoskeleton may function as a linear motor by differential and coordinated length-changes of the fibrils driven by conformational changes of the tetrameric subunits, the shape of which changes from near circular to elliptical. The cytoskeleton controls both the dynamic helical shape and the consequent motility of the cell. A stable cluster of proteins co-purifies with the cytoskeleton. These apparent membrane and membrane-associated proteins may function as anchor proteins.